This invention relates to the removal of thiols (mercaptans) from petroleum streams. Specifically, mercaptans of the five-carbon molecular weight range and above can be removed from petroleum streams. Removal of light thiols (less than C5 molecular weight), an enhancement to base assisted extractive processes such as extractive Merox(copyright), may also be improved.
To prepare fuels, which satisfy regulatory sulfur limits, it is necessary to process the fuels to remove various sulfur species. For example, long chain mercaptans are not native to crude, but are produced during the hydrotreatment of olefin-containing petroleum streams to remove sulfur species such as thiophenes. The byproduct, hydrogen sulfide, from the hydrodesulfurization process reacts with olefins present in the feeds to produce longer chain, higher molecular weight mercaptans. Normally, short chain (less than C5) mercaptans are easily and cheaply removed from such streams by base assisted extractive processes such as extractive Merox(copyright). However, due to the insolubility of the longer chain mercaptans in caustic, the normal extractive process is less effective. In the extractive process, the thiols are extracted from the petroleum stream into an aqueous caustic solution in the absence of air. The aqueous and petroleum streams are then separated. Once isolated from the petroleum stream, the extracted mercaptans in the aqueous stream are then catalytically oxidized with air and converted to disulfides. These disulfides are separated from the aqueous stream and disposed of into a waste stream. The limitation to this process is the solubility of the thiol in aqueous caustic. Thiols with chain lengths beyond five carbons are too oleophilic to be extracted into the aqueous phase and therefore cannot be fully removed by this process.
A large body of art exists in the patent literature describing additives used in conjunction with aqueous base to overcome the limitation due to the insolubility of long-chain mercaptans. All of these additives are added in substantial quantities ( greater than 10 wt % of aqueous phase) in order to modify the xe2x80x9csolvent powerxe2x80x9d of the caustic solution. In more modem terminology, these additives alter the solvent parameters of the aqueous caustic. The additive""s impact on solvent properties are proportional to the quantity added and therefore substantial quantities of additive are required to produce the substantive change required. In the literature these are commonly referred to as xe2x80x9csolubilizing agentsxe2x80x9d or xe2x80x9csolutizers.xe2x80x9d For example U.S. Pat. No. 2,059,075 describes the addition of xe2x80x9csubstantialxe2x80x9d amount of quaternary ammonium hydroxide to aqueous caustic to enhance mercaptan extraction. Other agents such as propyleneglycol (U.S. Pat. No. 2,183,801), butyleneglycols (U.S. Pat. No. 2,152,166), triethyleneglycol (U.S. Pat. No. 2,212,105) have been cited. In the ethyleneglycol family of additives, species containing greater than six carbons were noted as being xe2x80x9cunsuitablexe2x80x9d. Typically the preferred range of use for these solubilizers is from 25-75 wt % relative to the aqueous caustic. The use of such large quantities of expensive reagents and attendant problems of separation from extracted petroleum, undesirable decomposition and byproducts at operating conditions, etc, in using such large quantities, have precluded their widespread use in commercial practice.
One of these classes of additives, quaternary ammonium halides, has been found to be effective in low concentration for a sweetening process when used in conjunction with oxygen, oxidation catalyst and alkali metal hydroxide (U.S. Pat. No. 4,124,493). Subsequent patents (U.S. Pat. Nos. 4,156,641 4,206,079, 4,290,913 and 4,337,147) disclose the use of quaternary ammonium hydroxides in conjunction with a mercaptan oxidizing catalyst as components of solid oxidation catalyst composites to be used in the presence of oxygen for sweetening applications.
Another approach to reducing the sulfur content of petroleum streams has been to conduct bulk solvent extraction on the stream, such as is described in U.S. Pat. No. 2,792,332. This approach leads to losses of 20% of the original feed volume, which is unacceptable in many cases.
Other mercaptan removal or destruction processes are available, however, they remove sulfurs at the cost of saturating olefins, thereby decreasing the octane of the fuel being produced. For example, non-selective high-pressure catalytic hydrodesulfurization can be used to hydrogenate all olefins and ultimately reduce mercaptans but at a very high-octane loss.
Thus, what is needed in the art is a process for removing mercaptans, especially xe2x89xa7C5+ mercaptans, while maintaining octane.
The instant process describes a method for removal of mercaptans from petroleum streams comprising the steps of:
(a) extracting said petroleum stream, in the substantial absence of oxygen, with an aqueous medium comprising an aqueous base and a catalytically effective amount of a phase transfer catalyst or an aqueous solution of a catalytically effective amount of a basic phase transfer catalyst to remove said mercaptans from said petroleum stream;
(b) Separating and recovering an aqueous stream containing mercaptide anions and a petroleum stream having a reduced amount of mercaptans, and wherein when said phase transfer catalyst is a quaternary ammonium hydroxide, said quaternary ammonium cation has the formula: 
where q=1/w+1/x+1/y+1/z and wherein qxe2x89xa71.0 and wherein, Cw, Cx, Cy, and Cz represent alkyl radicals with carbon chain lengths of w, x, y and z carbon atoms respectively.
The process may also comprise steps of:
(c) subjecting said aqueous stream to oxidation to convert mercaptide anions contained therein to disulfides;
(d) separating said disulfides and recovering an aqueous stream having disulfides removed therefrom;
(e) recycling said aqueous stream to said step (a) wherein said aqueous stream contains said base and said phase transfer catalyst or said basic phase transfer catalyst of said step (a).